The present invention relates to the imaging of liquids flowing in heterogeneous objects using nuclear magnetic resonance (NMR) methods and, more particularly, to in vivo blood flow imaging wherein image contrast enhancement is achieved by exploiting blood flow rate to discriminate against the stationary media surrounding the blood flow network. In particular, novel methods are disclosed for producing a plan view image of blood flow in portions of the human body by providing two-dimensional phase-contrast images differentiating between a substantially-stationary body portion and a fluid flow velocity, or rate.
NMR imaging offers significant advantages as a medical diagnostic tool, the most important of which are (a) the completely non-invasive nature of the technology and (b) the ability to spatially encode the NMR signal data with a good degree of precision using field gradients. The term "Zeugmatography" has been coined recently to cover an increasing range of NMR techniques wherein static magnetic fields (to produce the polarization of nuclei) are combined with field gradients (to spatially encode the volume of interest) and with RF fields (to spatially reorient polarized nuclei) to achieve a wide range of objectives, including imaging. In the recent past, the technical and patent literature have burgeoned and have reported results of successive advances in the field. While the field has progressed steadily, certain intrinsic drawbacks have heretofore limited certain uses of NMR high resolution imaging in medicine. Chief among these are the comparatively slow nuclear spin relaxation times of human tissue, and body motion due both to inherent movements within the body and the difficulty of keeping the body stationary for long periods of time. Human tissue is known to have a spin-lattice relaxation time, T.sub.1, of approximately 0.5 seconds and a spin-spin relaxation time, T.sub.2, of approximately 0.05 seconds. Both of these time constants are very slow as compared to the speed of the instrumentation available to process NMR signals. Also, high resolution imaging requires a large number of pixel projections, each of which may be the result of a complete NMR pulse sequence, where each NMR sequence is at least influenced, if not limited, by these time constants. Therefore, real time (or even near-real time) imaging of body tissue has been of somewhat limited resolution, or contrast, and two-dimensional plan view maps of moving elements such as blood have heretofore only been discussed. High contrast two-dimensional imaging of in vivo blood flow in real time has been beyond the reach of NMR technologies.
Over the years, NMR has been used to measure flow, including flow rates in a variety of fluids as well as blood flow, but not in an imaging context. An early approach to using NMR in general for measuring fluid flow (actually liquid flow) is provided in U.S. Pat. No. 3,191,119 to Singer. This patent discloses the measurement of flow rates basically by measuring the amount of absorption energy needed to restore a transported volume of polarized liquid at a downstream location in a conduit, to the amount of polarization which was induced in an upstream location. While the disclosure recites applicability of the scheme to blood flow, it is clear that the apparatus is not conveniently adaptable to in vivo measurement. The Singer patent is illustrative of a fair-sized body of prior art using similar NMR techniques to measure liquid flow generally confined within conduits, around which instrumentation is placed. Recent patents of this genre are U.S. Pat. No. 4,259,638 to Krueger et al. and U.S. Pat. No. 4,110,680 to Bergmann et al.
Various methods for in vivo flow-encoding using pulsed-gradient NMR have been proposed, but most of these methods are sensitive only to a limited range of flow velocities. An application Ser. No. 490,605, entitled "NMR Blood Flow Imaging", utilizing an in vivo technique for discriminating against stationary tissue, was filed on May 2, 1983, assigned to the assignee of the present application and is incorporated herein in its entirety by reference. U.S. Pat. Nos. 4,431,968 and 4,443,760, even though not concerned with flow imaging, are both assigned to the assignee of the present application and are each incorporated herein in their entireties by reference, especially as to their teachings of NMR imaging systems and basic techniques.
Producing blood flow images of various sorts also may be found in the patent literature, but these make extensive use of acoustic or other forms of energy. U.S. Pat. No. 4,205,687 to White et al. discloses the production of a color-coded television or CRT type display of a portion of a blood vessel obtained using a mechanically articulated transducer to cover the area of interest on the patient. This approach uses basic Doppler processing and produces a velocity/color CRT image. U.S. Pat. No. 4,182,173 to Papadofrangakis et al. and assigned to the instant assignee, also discloses a sonic Doppler technique for imaging portions of the body including blood vessels, and produces a real time measurement of flow velocity in selected regions of the patient. A B-scan CRT display is provided on which cross-sectional view data is presented.
A method of measuring in vivo blood flow using hard radiation is described in U.S. Pat. No. 4,037,585 to Gildenberg. The disclosure recites the use of X or gamma ray scanning of the cranium in successive layers or slices by a narrow beam, and the subsequent digital processing of the resultant signals to build a visual presentation of the slice under examination. Additional non-invasive blood flow measuring techniques are taught in U.S. Pat. No. 3,809,070 to Doll et al., and in U.S. Pat. No. 4,334,543 to Fehr.
Despite the significant amount of effort directed toward the tasks of imaging human tissue in general, and in particular to the imaging of blood, it remains highly desirable to provide a non-invasive, in vivo, real time, high contrast NMR fluid-flow imaging method, and especially a method which would allow a single data set to substantially-simultaneously generate conventional T.sub.1 -weighted images, T.sub.2 images and blood-flow images.